The Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard provides techniques and protocols for supporting broadband wireless access. The standardization thereof was started in the year of 1999, and the IEEE 802.16-2001 was approved in 2001.
This is based on a single carrier physical layer called ‘WirelessMAN-SC’. In the IEEE 802.16a standard approved in 2003, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ were added to the physical layer, in addition to ‘WirelessMAN-SC’. The IEEE 802.16-2004 standard, revised after the completion of the IEEE 802.16a standard, was approved in 2004. In order to correct bugs and errors of the IEEE 02.16-2004 standard, the IEEE 802.16-2004/Cor1 (hereinafter, referred to as IEEE 802.16e) standard was completed in the form of ‘corrigendum’ in 2005.
Currently, in order to provide coverage extension and throughput enhancement in the IEEE 802.16 Task Group j (hereinafter, referred to as IEEE 802.16j) based on the “IEEE 802.16e, a relay station has been adopted, and the standardization thereof is in progress.
The use of relay stations in a wireless communication system may be contributive to expanding cell coverage and enhancing transmission performance.
In a case where a base station (BS) provides service to user equipment located outside the coverage of the base station by the use of a relay station, the relay station is made to relay all of control and data signals between the user equipment and the base station. In such a manner, the effect of cell coverage extension can be obtained.
In a case where user equipment is located within the coverage of a base station, a relay station amplifies a data signal between the base station and the mobile station and transmits it to each reception end, so that the user equipment can more stably communicate with the base station, thus enhancing transmission performance. A relay station may be required particularly when user equipment within the coverage of the base station is located in a shadow zone.
That is, in the IEEE 802.16j standard, a relay station is used to enable signal transmission to user equipment outside the coverage of a base station, and to allow user equipment within the coverage of the base station to set a high-quality channel with adaptive modulation and coding (AMC), so that system capacity can be increased with the same amount of radio resources.
As for representative relay modes by which a relay station relays signals between a base station and user equipment, there are an Amplify and Forward (AF) mode, and a Decode and Forward (DF) mode. Here, the AF mode is associated with amplifying a signal received from a base station or user equipment and then transmitting the amplified signal to the user equipment or the base station. In the DF mode, a signal received from a base station or user equipment is demodulated and decoded to recover information, coding, modulation and the like are then performed to generate a signal, and the signal is transmitted to user equipment or a base station.
The AF mode has advantages of short time delay in signal transmission but disadvantageously brings about the propagation or amplification of noise in transmission signals. Meanwhile, the DF mode can remove noise contained in a signal and increase the reliability of a transmission signal, but the demodulation and decoding may cause time delay in signal transmission.
Meanwhile, the development of wireless communication systems and the emergence of various services have continuously increased the demand for radio resources. This increases demand relative to supply of frequency resources, which causes a shortage of frequency. However, despite the shortage of frequency resources, the current frequency use is not efficient in most cases. For example, the actual efficiency of frequency use, measured as part of the Spectrum policy Task Force of FCC, is known to be 30% or less on average.
Accordingly, a cognitive radio communication system based on a Cognitive Radio (CR) scheme has been introduced as a technique that enables the efficient use of frequency resources not being used. This cognitive radio technique refers to a radio technique of determining radio transmission parameters such as frequencies, demodulation schemes, output and the like by sensing the surrounding environment. According to the cognitive radio technique, an unoccupied frequency is searched for according to area and time, thus protecting licensed users while enabling communication between non-licensed users.
In the basic cognitive radio communication system as above, spectrum sensing for non-licensed users, namely, secondary users (SU) is performed to avoid damage to licensed users, namely, primary users (PU). In such a manner, a frequency band that is not in use by the primary users is searched for, and resources in the band are used.
Only when a specific band is completely vacant, the secondary users may use a frequency within the band. Besides that, even if a frequency band is being used by the primary users, both the secondary and primary users may simultaneously use the same frequency band to such an extent that does not affect the primary users. In this simultaneous use, the secondary users may use the same resources as the primary users within the range in which the interference affecting the primary users is below an interference threshold.
In this cognitive radio communication system, to increase capacity for secondary users, many methods have been introduced in the range that maintains such an interference threshold.
However, if there are multiple relay stations that are available, the quality of communication services can be varied according to which relay station is selected. In particular, in the radio environment where primary and second users coexist, there is a need for establishing criteria of selecting an appropriate relay station in due consideration of the influence of transmission power, interference, and the like.